There are increasing cases in recent years where electric motors are adapted to driving systems that use inverters of pulse width modulation method (hereinafter referred to as PWM method). In the case of inverter driving such as the PWM method, there appears a potential difference between an outer ring and an inner ring of a bearing (hereinafter referred to as a shaft voltage) since an electric potential at a neutral point of a coil winding does not become zero. The shaft voltage includes high frequency components due to switching, and a small electric current flows in the bearing that causes electrolytic corrosion inside the bearing when the shaft voltage reaches a dielectric breakdown potential of an oil film in the bearing. A phenomenon of wave-like wear occurs in the inner ring, the outer ring and bearing balls of the bearing when the electrolytic corrosion progresses, and this gives rise to the possibility of abnormal noise, which becomes one of the major causes of problem in the electric motor.
Note that a power supply circuit for a drive circuit (including a control circuit and the like) that drives the electric motor with an inverter of the PWM method has been so constructed that it is electrically isolated from a primary circuit of the power supply circuit and an earth connection to the ground in the primary circuit side.
Shown below are some of the measures hitherto contrived to retard the electrolytic corrosion:
(1) by maintaining electrical continuity between an inner ring and an outer ring of the bearing;
(2) by isolating electrically between the inner ring and the outer ring of the bearing; and
(3) by reducing the shaft voltage.
Specific methods of achieving the above measure (1) include the use of electrically conductive lubricant for the bearing. However, the electrically conductive lubricant has problems such as deterioration of the conductivity with time, lack of the sliding reliability and the like. Also conceivable as the method of maintaining the electrical continuity is to install a brush on a rotary shaft. But this method still has other problems such as needing an additional space and dust produced from worn brush.
Specific methods relevant to the above measure (2) include the use of non-conductive ceramic balls to replace iron balls in the bearing. While this method is very effective to retard the electrolytic corrosion, it has a problem of high cost and not practical for application to the general-purpose motors.
One of the specific methods thitherto known to achieve the above measure (3) is to change an electrostatic capacitance and reduce the shaft voltage by electrically shorting a stator core and an electrically conductive metal bracket (refer to patent literature 1 for example). Among the conventional techniques for retarding the electrolytic corrosion in the bearing of electric motor, there are many structures disclosed, in which the stator core and the like of the electric motor are connected electrically with the earth ground (refer to patent literature 2 for example).
Incidentally, an electrostatic capacitance and a resistance connected in parallel has an impedance given by the expression of Z=1/jωC+R, where Z is the impedance, j is an imaginary number, ω is an angular frequency, C is the electrostatic capacitance and R is the resistance. As is known from this expression, the impedance decreases when the electrostatic capacitance increases or the resistance decreases. Conversely, the impedance increases when the electrostatic capacitance decreases or the resistance increases.
In patent literature 1, an impedance on the stator side is decreased by shorting the stator core and the bracket, and thereby reducing the electrolytic corrosion of the bearing.
In other words, electric motors used in water-related products such as washing machines and dishwasher-dryers that pose potential risk of electric shock are generally required to have an independent insulation (hereinafter referred to as supplementary insulation) in addition to the insulation for live parts (i.e., basic insulation). On the other hand, electric motors used for indoor units and outdoor units of air conditioners, water heaters, air cleaners and the like do not require any supplementary insulation since they are unlikely to pose the risk of electric shock. Accordingly, the motors used for indoor units and outdoor units of air conditioners, water heaters, air cleaners and the like have low impedance on the rotor side (i.e., inner ring side of the bearing) since their rotors are not provided with insulated structure. On the other hand, they have high impedance on the stator side (outer ring side of the bearing) because of their insulated structure. In this case, it is likely that a high shaft voltage appears due to an unbalanced condition resulting from a high potential of the inner-ring side of the bearing as compared to a low potential of the outer-ring side of the bearing. It is the shaft voltage of such a high potential that raises the possibility of developing electrolytic corrosion in the bearing.
To avoid such a condition, the method adopted in patent literature 1 eliminates an electrostatic capacitive component between the stator core and the bracket by shorting them, thereby reducing the impedance of the stator side (outer-ring side of the bearing) as discussed above and approximating it to the impedance of the rotor side (inner-ring side of the bearing).
Also proposed recently is a molded motor with improved reliability, in which fixing members such as a stator core at the stator side are molded with a molding material. It is conceivable here that the bearing is fixed with an insulation molding material in lieu of a metal bracket to prevent an undesirable high-frequency voltage from being generated on the outer-ring side of bearing and suppress an unwanted high-frequency current that flows between the outer ring and the inner ring of the bearing. There exist problems, however, because any of such molding materials is a synthetic resin that it has strength not sufficient to secure the bearing, poor accuracy of dimensions attributable to resin molding, and it is prone to a trouble of creeping in the bearing. In other words, it is normally likely that a shaft-bearing member such as this bearing exerts a force on the shaft in the radial direction due to a load being transmitted if there is a gap, for instance, between the outer ring and an inner surface of a housing. A slipping phenomenon tends to occur due to a relative difference in the radial direction when such a force is exerted, and that this slipping phenomenon is called creeping. In general, creeping of this nature can be suppressed by securely fixing the outer ring to the housing such as a bracket. It also becomes necessary to fix the bearing more securely to cope with the recent trend toward higher power output of electric motors. It is indispensable for this purpose to take measures against creeping, such as preparing beforehand a metal bracket formed of a steel plate having dimensions of high preciseness for fixation of the bearing. A general structure of the bearing, in particular, is to support the rotary shaft at two positions, and it is preferable to fix two bearings with the metal bracket for the reason of robustness and ease of embodiment as discussed here.
Some problems exist, however, with the conventional method shown in patent literature 1 as follows. That is, this conventional method precludes adjustment of impedance since it is the method of electrical shorting, which tends to cause a high shaft voltage depending on a magnet material and a structure of the rotor. Another problem to be cited is the need to always maintain the balance of electric potentials at their high levels between the inner ring and the outer ring of the bearing since it is the method of decreasing the impedance. Consideration has been given, as a possibility under such a condition that there is a case where electrolytic corrosion becomes liable when the shaft voltage increases contrary to the intention as a result of imbalance in the impedance attributed to a use environment of the motor, a deviation in the precision of assembling the stator and the rotor, and the like.
When a metal bracket is used for the reason of robustness as discussed above, there exists another possibility that impedance on the stator side decreases as compared with the structure of fixing the bearing by using the molding material such as an insulation resin. The possibility that has been considered is a case where the bearing becomes prone to electrolytic corrosion when the metal bracket is used because it reduces an insulation property and allows an electric current to flow between the inner ring and the outer ring of the bearing, whereas a resin housing keeps a condition of prohibiting the electric current from flowing between the inner ring and the outer ring of the bearing due to its high insulation property. The use of a conductive bracket has also posed a problem similar to that of patent literature 1 since it decreases impedance of the stator, which increases electric potentials of both the inner ring and the outer ring of the bearing.
As discussed above, the structure related to the problems addressed in this patent application is so constructed that a power supply circuit of the drive circuit (including a control circuit and the like) for driving the electric motor with an inverter of the PWM method is electrically isolated from a primary circuit of the power supply circuit and an earth connection to the ground in the primary circuit side. It is therefore difficult in view of specifications and characteristics of the motor besides other problems requiring consideration to aim at resolving the problems by employing any of structures of the conventional art of electrically connecting the stator core and the like of the motor to the earth ground, in addition to associated structures.
PTL 1: Japanese Patent Unexamined Publication, No. 2007-159302
PTL 2: Japanese Patent Unexamined Publication, No. 2004-229429